If you like SG1, you will like Atlantis. They’re both the same level of fun sci-fi.
I personally loved SGU. It was spiritual kin to the first two shows with the same thoughtful sci-fi world building, but it dialed the whimsy down to 1 and the drama up to 7. But it was still a Stargate show in every way. It respected established canon while building it’s own lore independent of the other shows. I was legit mad when it was canceled.
Yeah, it’s a definite departure in tone. But stick with it. It becomes a more serious rendition of the franchise, but it’s just as engaging.
It not only keeps the overall spirit of the franchise going, it revitalizes it because of the set and setting. Where SG1 and Atlantis have humanity becoming virtually indestructible gods through their new technology, EVERYONE in SGU is vulnerable.
It’s really unfortunate because the show was canceled right when it was finding it’s feet.